1. Field of the invention:
The present invention relates to drives for moving tooling and other loads which are normally present in manufacturing environments, and more particularly to drives incorporating brake and clutch mechanisms which selectively interface with one or more electric motors. Still more particularly, the present invention relates to an electrical actuation system for a drive in which clutching, braking or a combination thereof is effected by direct electromagnetic compression of an oil shear disc stack, wherein the excitation current of the electromagnetic coil is minimized because the working air gap is eliminated during actuation. More particularly still, the present invention relates to a drive of the class aforesaid, in which an armature assembly includes compression springs for supplying a predetermined level of compressive force on the disc stack while the working air gap is zero, independent of wear of the disc stack over the normal life thereof.
2. Description of the Prior Art
Movement of tools and other machinery in manufacturing environments is facilitated by drives which permit both rapid movement and slow movement, coupled with a brake function. Such drives permit rapid job cycling with accurate load positioning, while providing a maximum expected operational life of the drive. Typically, such drives interface with position locating apparatus which convert rotation into translation, such as by a ballscrew apparatus. Drives of this type incorporate two electric motors: a primary drive motor used for rapid movement and a secondary drive motor coupled through a gear reduction unit used for slow movement. These drives further incorporate a brake for precisely stopping movement and a clutch for selectively engaging the primary and secondary drive motors. The clutch is structured in the form of interleaved annular discs of two annular disc sets which collectively form a stack, in which one set of annular discs is attached to a shaft connected with the secondary drive motor, while the other set of annular discs is attached to a shaft connected with the primary drive motor, and a mechanism is used to selectively control clamping pressure between adjacent annular discs. The brake is structured analogously to the clutch, except that one annular disc set is attached to a stationary housing component.
The disc stacks utilized in the brake and clutch rely upon transmission of torque from one set of annular discs to the other set of annular discs. Some systems rely upon dry frictional engagement between adjacent annular discs to provide torque transfer, this is ordinarily considered unacceptable because of excessive wear and tendency for the dry friction material coating the annular discs to inconsistently rub relative to each other, resulting in a "stick-slip" jerking action which makes accurate positioning almost impossible to achieve. Systems which rely on dry frictional engagement between the annular disc sets generally are those which utilize low clamping force actuation systems, such as those which are electromagnetically operated. Accordingly, most conventional systems rely upon disc stacks utilizing an oil shear principle, in which adjacent annular discs brought into proximity by a clamping actuator, such as a pneumatic device, mutually transmit torque by a thin film of oil therebetween due to viscous shear of the oil film. The oil serves to reduce annular disc wear, provides consistent force transmission and conducts away waste heat. Accordingly, oil shear operated disc stacks have become industry standard, although there is involved a high degree of cost and installation complexity.
An example of a conventional two-speed drive is described in U.S. Pat. No. 4,463,841 to Kelley, dated Aug. 7, 1984. In this drive, a secondary electric motor drives through a gear reducer to an input shaft which connects with a clutch. The clutch is composed of an oil shear operated disc stack, in which one annular disc set is connected with the input shaft and the other annular disc set, interleaved with the first annular disc set, is connected with an output shaft. The output shaft connects with a primary electric motor, which, in turn, provides an output shaft for the drive. The output shaft interfaces with a brake composed of a second oil shear operated disc stack, in which one annular disc set is connected with the output shaft and the other annular disc set, interleaved with the first, is connected with the housing. A pneumatically operated bi-directional piston is axially moved to selectively apply annular disc clamping pressure to either the clutch disc stack or the brake disc stack. In the former mode, the secondary electric motor is able to drive the output shaft, while in the latter mode, rotation of the output shaft is braked. In the neutral position of the bi-directional piston, the brake is off, the secondary electric motor unconnected, and the primary electric motor is used as the prime mover. Selective movement of the bi-directional piston in concert with selective actuation of the electric motors enables an operator to achieve rapid and precise relocation of a load.
While the drive described in U.S. Pat. No. 4,463,841 operates acceptably in many situations, it has several significant problems which have been only partly addressed in the prior art.
One problem is that in the event of a power or fluid pressure failure, it is possible for the drive to continue spinning without the benefit of a brake. This problem was at least partly solved by a drive described in U.S. Pat. No. 4,607,736 to Kelley, dated Aug. 26, 1986, in which the brake stack is nominally clamped by biasing action of springs on the bi-directional piston, and which biasing action is overcome as long as fluid pressure is maintained in the pneumatic lines. This solution pertains, accordingly, only to pressurized fluid operated brake systems.
Another problem is that an external supply of pressurized fluid must be provided to actuate the bi-directional piston. This problem was solved by a drive described in U.S. Pat. No. 4,739,865 to Yater et al, dated Apr. 26, 1988, which discloses a drive incorporating clutch and brake components as generally described above and further incorporating a self-contained hydraulic pump system for actuating the bi-directional piston that controls clamping of the disc stacks. Accordingly, this drive eliminates the need for an external pressurized fluid source. However, this drive has the added costs of an internally provided pressurized fluid source, and it is not suited for use in two speed drives as it requires a motor to be running at the input shaft in order to actuate the bi-directional piston.
An improved drive is that disclosed in co-pending application Ser. No. 07/846,546 filed on Mar. 5, 1992, hereby incorporated by reference. Therein disclosed is a two-speed drive which utilizes an electromagnetically operated control system which is wired with the respective drive motors, is automatically braked in the event of a loss of power, is provided with clutch and brake disc stacks which are pre-assembled, and is provided with a manual over-ride which allows for manual operation.
The two-speed drive includes a primary drive motor which is connected at one end to an external drive shaft and at an opposite end to a primary clutch shaft. The primary clutch shaft is connected with a first set of annular discs of an oil shear clutch disc stack. A secondary clutch shaft is connected with a second set of annular discs of the clutch disc stack. The clutch disc stack is biased by springs so as to be in a nominally clamped mode, which is released by operation of a clutch electromagnetic coil assembly. The secondary input shaft is connected through a gear reduction unit to a primary input shaft that is connected at one end to a secondary drive motor and at the other end to a first set of annular discs of an oil shear brake disc stack. A second set of annular discs of the brake disc stack is connected to a stationary housing component. The brake disc stack is biased by springs to be in a nominally clamped mode, which is released by operation of a brake electromagnetic coil assembly. The clutch and brake disc stacks are pre-assembled and held in a pre-aligned orientation by operation of a plurality of alignment pins and wave washers carried on the alignment pins. A manual over-ride is provided adjacent the brake electromagnetic coil assembly which permits selective release of spring biasing with respect to the brake disc stack so that a manual crank may be operated.
High speed positioning is accomplished by energization of the primary drive motor as well as energization of a primary electromagnetic coil of the clutch electromagnetic coil assembly. Braking is achieved by de-energizing the primary drive motor and the primary electromagnetic coil. Low speed positioning is accomplished by energization of the secondary drive motor (while the primary drive motor and primary electromagnetic coil are de-energized) and energization of a secondary electromagnetic coil of the brake electromagnetic coil assembly. Braking is achieved by de-energizing the secondary drive motor and the secondary electromagnetic coil.
It is well known that magnetic reluctance of a magnetic circuit is many times larger for an air gap of any size than for the case of a magnetic circuit having a zero air gap. Consequently, large excitation currents are needed to produce a desired level of magnetic field strength, whereas much smaller excitation currents are needed to produce the same magnetic field strength when there is no air gap in the magnetic circuit. The amount of current is important because heating losses are proportional to the square of the current.
As a consequence, it is conventional practice, as exemplified by the devices described hereinabove, to utilize springs to provide clamping of the disc stack, while the electromagnetic coil is used to overcome the spring pressure to thereby disengage clamping of the disc stack. That is, the electromagnetic coil is not used to directly supply clamping force to the disc stack, but rather to disengage clamping of the disc stack. The reason for this is that over time the discs of the disc stack will wear, so that a predetermined movement of an armature relative to an electromagnet pole piece by which is provided a zero air gap may initially provide a correct clamping force, but over time will not provide the correct clamping force. Ergo, since compressed springs can accommodate disc wear, conventional electromagnetically actuated drive systems use compressed springs to supply clamping force to the disc stack, and use electromagnetics to disengage the spring provided clamping force, primarily because the disengagement movement of the armature is not wear sensitive and so the air gap can, in this mode of operation, go to zero.
Accordingly, there is no practical means known in the present art to provide an electromagnetic actuation system for a drive in which the electromagnet directly supplies clamping force to the disc stack.